Method for Forming a Nickelsilicide FUSI Gate

ABSTRACT

Ni 3 Si 2  FUSI gates can be formed inter alia by further reaction of NiSi/Ni 2 Si gate stacks. Ni 3 Si 2  behaves similarly to NiSi in terms of work function values, and of modulation with dopants on SiO 2 , in contrast to Ni-rich silicides which have significantly higher work function values on HfSi x O y  and negligible work function shifts with dopants on SiO 2 . Formation of Ni 3 Si 2  can applied for applications targeting NiSi FUSI gates, thereby expanding the process window without changing the electrical properties of the FUSI gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 12/037,486 filed Feb. 26, 2008, which in turn claims priority to U.S. Provisional Patent Application Ser. No. 60/892,074 filed Feb. 28, 2007.

BACKGROUND

The present disclosure relates to semiconductor process technology and devices. In particular, it relates to semiconductor devices with metallic gate electrodes formed by a reaction between a metal and a semiconductor material.

Metal gate electrodes are expected to be used in future complementary metal-oxide semiconductor (CMOS) devices in order to address depletion issues of a semiconductor gate electrode and to promote compatibility of the gate electrode with high-k dielectric materials of a gate dielectric. High-k dielectrics are dielectric materials having a relative dielectric constant higher than that of SiO₂, typically above 10. For these metal gate electrodes, work function (WF) values of about 4.2 eV and about 5 eV are respectively used for obtaining n-type and p-type gate electrodes in corresponding NMOS and PMOS devices. Metal gate electrodes can be formed by full silicidation (FUSI) of an initial semiconductor gate electrode with a metal in contact therewith. This semiconductor material may be polycrystalline silicon. The metal may be a refractory metal such as W, a noble metal such as Pt, a near noble metal such as Ni, a transition metal such as Ti, or any combination thereof. During this silicidation process, the initial semiconductor gate electrode reacts with the metal and is thereby fully converted into a semiconductor-metal alloy, known as silicide.

Nickel (Ni) fully silicided (FUSI) gate electrodes have recently attracted attention as candidates for metal gate electrodes. Initial studies focused on Ni₁Si₁ (or NiSi) and the ability to modify the effective work function of NiSi FUSI gate electrodes when formed on SiO₂ or SiON gate dielectrics with dopants to obtain either an n-type or p-type work function. Elements such as As, P or B are then incorporated in the NiSi gate electrode. It was found that dopants implanted into the polysilicon gate electrode before silicidation segregate during silicidation to the interface between the gate electrode and the gate dielectric. This dopant segregation resulted in a significant pile-up of the dopants at this interface after full silicidation.

Recently, the focus shifted to include other Ni silicide phases such as Ni₂Si and Ni₃₁Si₁₂ to form a fully silicided gate electrode. Compared to a NiSi fully silicided gate electrode, these other Ni silicide phases, with a ratio Ni to Si that is higher than 1, were found to yield a fully silicided gate electrode with a higher work function compared to a NiSi fully silicided gate electrode when formed on a HfSi_(x)O_(y)N_(z) dielectric layer. These Ni-rich phases are thus more attractive for fabricating p-type gate electrodes. However, these other Ni-rich silicide phases exhibited no significant work function modulation with dopants when formed on a SiO₂ dielectric layer, despite the fact that a similar interfacial pile-up of dopants was observed on the interface between a Ni-rich silicided gate electrode with the gate dielectric in case a SiO₂ or HfSi_(x)O_(y)N_(z) was used as a dielectric material.

CMOS integration schemes using different Ni silicide phases for respectively n-MOS, e.g. NiSi, and p-MOS, e.g. Ni₂Si or Ni₃₁Si₁₂, taking advantage of the differences in work function of these nickel silicide phases on HfSi_(x)O_(y)N_(z), have been demonstrated. J. Kittl et al. discloses in “CMOS Integration of Dual Work Function Phase-Controlled Ni Fully Silicided Gates (NMOS: NiSi, PMOS:Ni₂Si and Ni₃₁Si₁₂) on HfSiON”, IEEE electron device letters Vol. 27, No 12 (December 2006), p. 967-968, such an integration scheme for fabricating CMOS devices with different work function for NMOS and PMOS device using different nickel silicide phases for each device type.

The substantial difference in work function among these Ni silicide phases when formed on an HfSi_(x)O_(y)N_(z) gate dielectric, and in their work function dependency on doping when formed on a SiO₂ gate dielectric also implies the need to control the silicide phase formation in order to ensure good control and uniformity of the resulting device threshold voltages. J. Kittl et al discloses in “Work Function of Ni Silicide Phases on HfSiON and SiO2: NiSi, Ni₂Si, Ni₃₁Si₁₂ and Ni₃Si Fully Silicided Gates”, IEEE electron device letters Vol. 27, No 1, January 2006, p 34-36, an optimized two-step Rapid Thermal Processing (RTP) nickel silicidation process wherein the reacted Ni to Si ratio was controlled by selecting the thermal budget of the first RTP step thereby allowing the control of nickel silicide phase obtained during the silicidation process.

The applicability of Ni fully silicided gates may be limited, among other factors, by the ability to control the reacted Ni-to-Si ratio and hence to control the resulting silicide phase present at the interface with the gate dielectric, where the silicide phase determines the gate electrode work function. Furthermore, although NiSi is used to fabricate fully nickel silicided gate electrodes, the process window, in terms of as-deposited Ni-to-Si ratio and/or thermal budget of the silicidation process, for forming NiSi is limited.

SUMMARY

Methods of fabricating a nickel fully-silicided-gate electrode device are disclosed. One such method comprises: providing a MOSFET device (1) having a silicon gate electrode (5) formed on a gate dielectric (4); providing a nickel layer (12) in contact with the semiconductor gate electrode (5); and performing a silicidation process to form Ni₃Si₂ silicide (15) at least at the interface between the gate electrode (5) with the gate dielectric (4).

Another method of fabricating a nickel fully-silicided-gate electrode device, is disclosed. The method comprises: providing a MOSFET device (1) having a silicon gate electrode (5) formed on a gate dielectric (4); providing a nickel layer (12) in contact with the semiconductor gate electrode (5); and performing a silicidation process to form Ni₃Si₂ silicide (15) at least at the interface between the gate electrode (5) with the gate dielectric (4). In this embodiment, the silicidation process comprises: performing a first thermal process step to have the semiconductor gate electrode (5) react with the provided nickel (12) thereby forming a gate electrode stack comprising NiSi (13) and a metal-rich silicide phase (14), preferably Ni₂Si, selectively removing unreacted nickel (12); and performing a second thermal process step to have the gate electrode stack comprising NiSi (13) and a metal-rich silicide phase (14) converted into a Ni₃Si₂ silicide phase (15). The method can further comprise incorporating dopants such as P and/or B in the Ni₃Si₂ silicide phase (15) to modulate the work function thereof. Preferably the gate dielectric is silicon-oxide or a hafnium based dielectric such as hafnium-oxy-silicate.

Yet another method of fabricating a nickel fully-silicided-gate electrode device, is disclosed. This method comprises: providing a MOSFET device (1) having a silicon gate electrode (5) formed on a gate dielectric (4); providing a nickel layer (12) in contact with the semiconductor gate electrode (5); and performing a silicidation process to form Ni₃Si₂ silicide (15) at least at the interface between the gate electrode (5) with the gate dielectric (4). This method can further comprise incorporating dopants such as P and/or B in the Ni₃Si₂ silicide phase (15) to modulate the work function thereof. Preferably the gate dielectric is silicon-oxide or a hafnium based dielectric such as hafnium-oxy-silicate.

A MOSFET (1) is disclosed comprising a gate electrode (5) and a gate dielectric (4), wherein the gate electrode (5) comprises at least Ni₃Si₂ (15) at least at the interface with the gate dielectric (4). The Ni₃Si₂ (15) of this MOSFET device (1) can further comprises dopants, such as P and/or B for modulating the work function thereof.

Preferred embodiments of the present disclosure provide a substitute material for NiSi as metal gate electrode, where the substitute material has a work function behavior comparable to NiSi.

Preferably, such a substitute can be formed in a process window adjacent to the process window for fabricating a NiSi metal gate electrode. In particular, it is preferable that process control requirements can be alleviated using embodiments in which a NiSi or NiSi-like metal gate electrode can be obtained over a larger range of process parameters.

In some embodiments, a stable nickel Fully Silicided gate electrode is provided using this NiSi substitute material at low processing temperatures.

Preferably a nickel fully silicided gate electrode is provided having an n-type work function on Hafnium-based gate dielectrics and a work function on silicon-oxide or silicon-oxy-nitride gate dielectrics that can be modulated by dopants.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of the drawings. The embodiments and figures disclosed herein must be considered illustrative rather than restrictive. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes and do not necessarily correspond to actual reductions to practice. Same numerals are used to refer to corresponding features in the drawings

FIG. 1 shows X-Ray Dffraction patterns (XRD) (Cu K_(α) radiation), intensity in Arbitrary Units (A.U.), versus angle 2Theta (deg), of silicide films formed by a single Rapid Thermal Processing (RTP1) reaction of 84 or 90 nanometer (nm) Ni with 100 nm poly-Si formed on a SiO₂ layer for different RTP thermal budgets, i.e. combinations of RTP1 temperature (° C.) and RTP time (s).

FIG. 2 shows Flatband Voltage (V_(FB)) (V) versus Equivalent Oxide Thickness EOT (nm) of undoped FUSI gate electrodes formed on HfSi_(x)O_(y)/SiO₂ gate dielectric stacks, for (a) NiSi, Ni₃Si₂,Ni₂Si and Ni₃₁Si₁₂, whereby the ratio Hf/(Hf+Si)=40 at. % with fixed HfSi_(x)O_(y) thickness and varying SiO₂ thicknesses; and (b) Ni₃Si₂, whereby the ratio Hf/(Hf+Si)=40 and 60 at. % with varying HfSi_(x)O_(y) thickness and fixed SiO₂ thicknesses.

FIG. 3 shows Flatband Voltage (V_(FB)) (V) versus Equivalent Oxide Thickness (EOT) (nm) for undoped NiSi, Ni₃Si₂, Ni₂Si and Ni₃₁Si₁₂ FUSI gate electrodes formed on a SiO₂ gate dielectric, and for P-doped NiSi and Ni₃Si₂ FUSI gate electrodes formed on a SiO₂ gate dielectric.

FIG. 4 shows Work Function (WF) (eV) of NiSi, Ni₂Si, Ni₃Si₂ and Ni₃₁Si₁₂ FUSI gate electrodes on SiO₂ (left) and HfSi_(x)O_(y) (right) gate dielectrics. The FUSI gate electrodes formed on a SiO₂ gate dielectric are doped showing the effect of P and B dopants on the work function of the gate electrode. The Ni₃Si₂ FUSI gate electrode is formed on HfSi_(x)O_(y) gate dielectrics with varying Hf content (Hf/(Hf+Si)=23, 40 and 60 at. %). The Hf content of the HfSi_(x)O_(y) gate dielectric for the NiSi, Ni₂Si and Ni₃₁Si₁₂ FUSI gate electrode was Hf/(Hf+Si)=40 at. %.

FIG. 5 shows the nickel silicide phase formation as function of the thermal budget of the second Rapid Thermal Processing (RPT2) step (temperature (° C.) for 30 s anneal time) and as function of ratio Ni-to-Si before silicidation, expressed as either thickness ratio (t_(Ni)/t_(Si)) or composition ratio (x/y).

FIG. 6 shows the nickel silicide phase formation as function of the thermal budget of the first RTP1 step (temperature (° C.) for 30 s anneal time) and as function of composition ratio Ni-to-Si (x/y).

FIGS. 7 a-d show schematic cross-sections of a device illustrating different steps in a fabrication process.

DETAILED DESCRIPTION

In the present disclosure, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. Such methods are known to a person skilled in the art.

In the framework of the present disclosure, the terms “silicide”, “silicided”, “silicidation” can refer to the reaction between a metal and silicon, but is not intended to be limited to silicon. For instance, the reaction of a metal with SiGe, Ge, or any other suitable semiconductor material, may still be referred to as silicidation.

For selected metal-semiconductor alloys, i.e. silicides, the work function thereof may depend on the specific phase in which the alloy is formed. Hence, the suitability of such metal-semiconductor combinations for use as gate electrode for one type of transistor depends on which phase of this metal-semiconductor combination can be formed for this type of transistor and/or if the work function of this phase can be modulated by incorporating dopants therein. This specific phase is to be formed at least at the bottom part of the gate electrode (the part nearest to the gate dielectric), referred to herein as the interface. This specific phase may be formed within the bottom few nanometers of the gate electrode, e.g., the last nanometer, or the last 2, 3, 4, 5, 10 nanometers or more.

In other words, in the context of the present disclosure, the term “interface,” when referring to the specific silicide phase of the gate electrode, refers to the bottom part of the gate electrode, which is the nearest to the gate dielectric, of few nanometers thickness, e.g. between about 1 nm and about 10 nm, preferably between about 1 nm and about 5 nm.

This specific silicide phase, also referred to as metal-semiconductor phase, can be represented by the formula M_(x)S_(y), wherein M represents the metal, S represents the semiconductor and wherein x and y are integers or real numbers different from 0. In a metal-rich silicide, x/y is larger than 1. The numbers x and y thus refer to the ratios of the reacted metal and semiconductor.

More particularly, with regard to nickel silicide, for metal-rich phases such as Ni₂Si, Ni₃Si₂, Ni₃₁Si₁₂, or Ni₃Si, the composition ratio x/y is higher than 1 and preferably less than or equal to about 3 (i.e. 1<x/y≦3), while for metal-poor phases such as NiSi or NiSi₂, the ratio x/y is higher than 0 and less than or equal to about 1 (i.e. 0<x/y≦1).

In the content of the present disclosure, the ratio nickel-to-silicon can be indicated as the composition ratio x/y or as the thickness ratio t_(Ni)/t_(Si) of the as-deposited nickel and silicon layers. The relationship between both ratios is the density of the respective layers. A thickness ratio nickel-to-silicon of 1 corresponds to a composition ratio of about 1.6.

In contrast to other stable low temperature nickel silicide phases, Ni₃Si₂ has received little attention to date. It can be formed as a stable phase in the normal phase sequence for Ni-to-Si composition ratios x/y between 1 and 2.

In the case of a composition ratio x/y of about 2, a stack of NiSi and Ni₂Si is formed first whereby NiSi is in contact with the gate dielectric and Ni₂Si is on top of the NiSi. Thereafter, upon applying thermal energy, Ni₃Si₂ will grow by the reaction NiSi+Ni₂Si→Ni₃Si₂. This reaction occurs at moderate thermal budgets, e.g. at a high enough RPT2 temperature or by using additional high enough thermal budget provided during subsequent processing. The conversion to Ni₃Si₂ will generally be driven to completion with thermal budgets used in back-end of line CMOS processing, such as at temperatures below 400° C. for sufficient time periods, e.g. 30 minutes, thereby fully consuming the reactant phase that is present initially in a smaller amount. Full consumption of the NiSi layer by Ni₃Si₂ formation defines in consequence the upper limit in Ni content incorporated in the gate electrode during the first silicidation step, e.g., limiting the first RTP (RTP1) thermal budget when Ni availability does not limit the reaction between silicon and nickel. Such abundance of nickel occurs in particular for small patterned structures. The first thermal budget of the silicidation process is therefore preferably chosen to form a stack of NiSi and Ni₂Si, the ratio between both silicide phases in the stack being determined by the thermal budget and nickel-to-silicon composition ratio: the higher the thermal budget of RTP1 and/or the more as-deposited nickel being available for silicidation during RPT1, the more Ni₂Si is formed compared to NiSi. Given the narrow process window found for NiSi FUSI gates, as shown in FIG. 6 (350° C.<RTP1<360° C. if NiSi is to be formed at the interface with the gate dielectric), it is important to understand what work function is expected if Ni₃Si₂ is formed at the interface with the dielectric. In addition, given the different behavior between NiSi and the Ni-rich silicides such as Ni₂Si, Ni₃₁Si₁₂ and Ni₃Si both on SiO₂ and on Hf silicates gate dielectrics, it is of interest to compare the behavior of Ni₃Si₂ to that of the other Ni silicides.

In this application, the characterization of the effective work function of Ni₃Si₂ on SiO₂ including the effect of dopants, and on HfSi_(x)O_(y) including the dependency on the Hf content of the hafnium-based dielectric is disclosed. In the embodiment illustrated by FIGS. 1 to 4, a single RPT silicidation process was used to form the Ni₃Si₂.

Ni FUSI capacitors were fabricated on (100) p-type Si. Dielectric thickness series were used for accurate work function extraction accounting for charge effects. A 10-nm high-quality thermal SiO₂ was first grown at 900° C. on 200 mm wafers using shallow-trench isolation to isolate active areas. A slant etching technique was then used, where SiO₂ is selectively wet etched to obtain a gradual thickness variation in the 4-7 nm range across each wafer. On selected wafers, HfSi_(x)O_(y) films of different thicknesses (2, 3, 4 and 6 nm) and different Hf contents (Hf/(Hf+Si)=23, 40 and 60 at %) were subsequently deposited by metal-organic chemical vapor deposition. Deposition of 100 nm polycrystalline silicon (poly-Si) followed, with selected wafers receiving either B (3 keV, 6×10¹⁵ cm⁻²) or P (8 keV, 4×10¹⁵ cm⁻²) ion implantation. All wafers received a 1050° C. spike anneal. Ni films in the 60-145 nm thickness range were then deposited and reacted by a single RTP1 step to form nickel silicides at temperatures in the 425-625° C. range. Unreacted Ni was then removed in a selective etch. Materials characterization included X-ray diffraction (XRD), and Rutherford Backscattering Spectrometry (RBS), scanning and transmission electron microscopy. The thus formed structures were subjected to a 420° C. 20 min forming gas anneal. Capacitance-voltage (C-V) measurements were performed on these annealed structures.

NiSi, Ni₂Si and Ni₃₁Si₁₂ FUSI gates were formed for deposited Ni thicknesses of 60, 120 and 145 nm on the 100 nm poly-Si and single RTP1 temperatures of 550° C., 425° C. and 480° C. respectively (30 s). Process conditions for formation of the FUSI gates were optimized to ensure the presence of the target phase as dominant phase at the dielectric interface.

Phase formation of Ni₃Si₂ is illustrated in FIG. 1. For 90 nm Ni films (Ni—Si composition ratio slightly >1.5) and single RTP1 of 450° C. 30 s, XRD peaks corresponding to NiSi (circles) and Ni₂Si (squares) are observed. The resulting silicide film is a stack of NiSi with Ni₂Si on top. For RTP1 of 550° C. 30 s, the XRD pattern corresponds to Ni₃Si₂ (diamonds). Small Ni₂Si peaks are also observed indicating the presence of this secondary phase (note the logarithmic scale on the ordinate axis of FIG. 1), as expected for the chosen Ni—Si ratio of slightly >1.5. NiSi peaks have disappeared indicating the Ni₃Si₂ formation has fully consumed this phase. We note that if a slightly Ni deficient ratio had been chosen (composition ratio x/y<1.5), NiSi could remain at the interface with the gate dielectric and the work function could be still controlled by this NiSi phase. Similar XRD patterns are observed for higher thermal budgets, e.g. at higher RPT1 temperatures (625° C., 30 s) or for longer furnace anneals at lower temperatures (425° C., 1 hr). The latter thermal budget is comparable to those used in CMOS back end processing and confirms that the reaction to form Ni₃Si₂ is driven to completion in this case even if an incomplete conversion into Ni₃Si₂ is obtained during the silicidation process. A smaller amount of the Ni₂Si impurity phase is observed when 84 nm Ni is deposited on the 100 nm poly-Si (Ni—Si composition ratio x/y slight <1.5) and reacted at an RTP1 of 575° C. 30 s.

C-V measurements were performed on Ni₃Si₂, NiSi, Ni₂Si and Ni₃₁Si₁₂ undoped FUSI capacitors formed on HfSi_(x)O_(y)/SiO₂ gate dielectric stacks. Equivalent Oxide Thickness (EOT) and Flat-Band voltages (V_(FB)) were obtained using the Hauser routine. Process conditions chosen for the Ni₃Si₂ capacitors were 90 nm Ni annealed at 550° C. 30 s. FIGS. 2 a-b show V_(FB) vs. EOT plots for these nickel silicide phases. FIG. 2( a) corresponds to fixed HfSi_(x)O_(y) thickness, Hf/(Hf+Si)=40%, and varying SiO₂ thickness; and FIG. 2( b) corresponds to fixed SiO₂ thickness and varying HfSi_(x)O_(y) thickness for Hf contents of 40 and 60%. The fixed charge density (Q_(f)) at the SiO₂/substrate interface, of ˜1×10¹¹ cm⁻² is similar for the different silicide phases as in FIG. 2( a) the slope of the V_(FB) vs. EOT is comparable for all different silicide phases. Charge contributions from the HfSi_(x)O_(y) films and the HfSi_(x)O_(y)/SiO₂ interfaces are negligible, as seen in FIG. 2( b), where charge effects affecting the V_(FB) vs. EOT plots are similar to those found when only the SiO₂ thickness is varied, indicating that the behavior is still dominated by fixed charges at the SiO₂/Si interface.

C-V measurements were performed on undoped Ni₃Si₂, NiSi, Ni₂Si and Ni₃₁Si₁₂ FUSI capacitors and on P-doped Ni₃Si₂ and NiSi FUSI capacitors. The FUSI capacitors are formed on a SiO₂ gate dielectric. Equivalent oxide thickness (EOT) and flat-band voltages (V_(FB)) were obtained using the Hauser routine. Process conditions chosen for the Ni₃Si₂ capacitors were 90 nm Ni annealed at 550° C. 30 s. FIG. 3 show V_(FB) vs. EOT plots for these phases. The flatband voltage of both Ni₃Si₂ and NiSi FUSI capacitors is shifted over about the same range by incorporating dopants in the silicide phase, thereby illustrating that the work function of a Ni₃Si₂ gate electrode can be modulated by doping thereof. Moreover the dependency of the work function of a Ni₃Si₂ gate electrode on doping is similar to a NiSi gate electrode.

Effective work function extracted from the thickness series are shown in FIG. 4. The work function of NiSi, Ni₂Si, Ni₃Si₂ and Ni₃₁Si₁₂ FUSI gates on SiO₂ (left) and HfSi_(x)O_(y) (right) dielectrics is plotted. When P or B doping NiSi and Ni₃Si₂ FUSI gate electrodes formed on a SiO₂ gate dielectric, the work function was shifted considerably in an almost similar way. For Ni₂Si and Ni₃₁Si₁₂ FUSI gate electrodes on a SiO₂ gate dielectric, the work function remains essentially unaffected by the P or B doping. The addition of dopants (P or B) before silicidation thus results in a modulation of the work function of Ni₃Si₂ on SiO₂ similar to the work function modulation observed for NiSi. In contrast, significantly reduced work function shifts with doping are seen for Ni-rich silicides.

The dependency of the work function of a Ni₃Si₂ FUSI gate electrode formed on a HfSi_(x)O_(y) gate dielectric on the Hf content of this gate dielectric is also plotted. The Hf content (=Hf/(Hf+Si)) was set at 23, 40 and 60 at. % for the Ni₃Si₂ FUSI gate electrode on HfSi_(x)O_(y). The Hf content for the NiSi, Ni₂Si and Ni₃₁Si₁₂ gates on HfSi_(x)O_(y) was set Hf/(Hf+Si)=40 at. %. The work function of Ni₃Si₂ is close to that of NiSi on HfSi_(x)O_(y) and has little dependence on the Hf content within the range studied. Higher work function values are seen for the Ni-rich silicides on HfSi_(x)O_(y). Again a similar behavior is observed for NiSi and Ni₃Si₂ FUSI gate electrode, in contrast to other Ni-rich silicides.

Since the Ni₃Si₂ phase grows after a NiSi/Ni₂Si stack is formed, with NiSi at the interface with the dielectric, the gate electrodes have initially a structure and work function corresponding to NiSi FUSI gates including the pile-up of dopants at the dielectric interface if dopants are used. The results presented here suggest that the structure of the initial NiSi gate electrode/gate dielectric interface is not significantly modified by the growth of Ni₃Si₂. Thus, a similar dipole layer, resulting from the pile-up of dopants at the dielectric interface, may be responsible for the work function modulation on SiO₂ of both NiSi and Ni₃Si₂ gate electrodes. From the perspective of CMOS integration of Ni FUSI gates, the similar work function values and behavior of NiSi and Ni₃Si₂ gate electrodes suggest that formation of Ni₃Si₂ may be applied for applications targeting NiSi-like FUSI gates, implying a relaxation of the process control constraints due to the larger process window when considering the formation of FUSI gates with either NiSi or Ni₃Si₂ at the dielectric interface relative to the NiSi-only case.

Ni₃Si₂ FUSI gate electrodes can be formed by further reaction of NiSi/Ni₂Si gate stacks. The work function of Ni₃Si₂ on SiO₂ and HfSi_(x)O_(y) behaves similarly to NiSi in terms of work function values and of modulation with dopants on SiO₂, in contrast to Ni-rich silicides, which have significantly higher work function values on HfSi_(x)O_(y) and negligible work function shifts with dopants on SiO₂. These results suggest that the electrode-dielectric interface is not modified significantly by the subsequent growth of Ni₃Si₂, maintaining similar characteristics to that of the original NiSi/dielectric interface, and imply that formation of Ni₃Si₂ may be tolerable for applications targeting NiSi FUSI gates, thereby expanding the process window.

As already indicated above, Ni₃Si₂ can be formed as a stable phase in the normal phase sequence for Ni-to-Si composition ratios x/y between 1 and 2. This is illustrated in FIG. 5 showing the nickel silicide phase formation as function of the thermal budget of the second RTP step (temperature (° C.) for 30 s anneal time) and as function of ratio Ni-to-Si, either thickness (t_(Ni)/t_(si)) or composition (x/y) before silicidation.

If the ratio Ni-to-Si is selected between 0.55<t_(Ni)/t_(Si)<1, (0.9<x/y<1.8), preferably between 0.6<t_(Ni)/t_(Si)<0.9, (1<x/y<1.6), then the thermal budget of the first temperature step (RTP1) of the silicidation process is preferably chosen to form a stack of NiSi and Ni₂Si, the ratio between both silicide phases in the stack being determined by the thermal budget and nickel-to-silicon ratio. The higher the thermal budget of RTP1 and/or the more as-deposited nickel being available for silicidation during RPT1, the more Ni₂Si is formed compared to NiSi. A method to select the nickel and silicon content of a silicided gate electrode by controlling the thermal budget of the first thermal step RPT1 of the silicidation process, is disclosed in United States patent application US 2006 0258156, hereby incorporated by reference in its entirety. If the thermal budget of the second temperature step (RTP2) of the silicidation process is selected to be high enough, e.g. RTP2>500° C. for 30 s, or temperature>425° C. for 60 minutes, the stack of NiSi and Ni₂Si is partially or completely converted into Ni₃Si₂ layer thereby forming a Ni₃Si₂ silicide phase at least at the interface with the gate dielectric.

As discussed above, Ni₃Si₂ FUSI gates can be formed by further reaction of NiSi/Ni₂Si gate stacks. Ni₃Si₂ behaves similarly to NiSi in terms of work function values, and of modulation with dopants on SiO₂, in contrast to Ni-rich silicides which have significantly higher work function values on HfSi_(x)O_(y) and negligible work function shifts with dopants on SiO₂. Formation of Ni₃Si₂ can be applied for applications targeting NiSi FUSI gates, thereby expanding the process window without changing the electrical properties of the FUSI gate.

The advantage of using Ni₃Si₂ as a substitute for NiSi is illustrated by FIG. 6, showing the nickel silicide phase formation as function of the thermal budget of the first RTP1 step (temperature (° C.) for 30 s anneal time) and as function of composition ratio x/y Ni-to-Si. The thermal budget of the second thermal step RPT2 of the silicidation process was selected above 500° C. 30 s. In the prior art, the nickel-to-silicon ratio x/y is selected between 1 and 1.5 while the thermal budget of the first thermal step RTP1 was between 350° C. 30 s and 360° C. 30 s to form the predominant NiSi phase controlling the electrical properties of the gate electrode. During the second thermal step, a Ni₃Si₂ phase can be formed upon the NiSi phase at the interface with the gate dielectric. This NiSi process window is indicated in FIG. 6 with the dotted triangle. However, if the nickel to silicon ratio x/y is selected between 1.5 and 2 while the thermal budget of the first thermal step RTP1 was between 360° C. 30 s and 375° C. 30 s, a predominant Ni₃Si₂ phase at the interface with the gate dielectric is formed controlling the electrical properties of the gate electrode 5. As illustrated above, the Ni₃Si₂ phase will have work function properties similar to those of NiSi. Hence, the dashed triangle in FIG. 6 illustrates a process window wherein a similar work function behavior for a nickel fully silicided gate can be obtained as for a NiSi FUSI gate. This process window is adjacent to the process window for obtaining NiSi as predominant silicide phase determining the work function of nickel silicide. At the upper end of this range, illustrated by the dashed triangle, another metal rich phase Ni₂Si may be formed upon the predominant Ni₃Si₂ phase in contact with the gate dielectric. Above this range metal-rich phases such as Ni₂Si or Ni₃₁Si₁₂ will dominate the work function of the FUSI gate electrode thereby showing different work function properties compared to NiSi or Ni₃Si₂.

A method for fabricating a nickel fully-silicided-gate device is disclosed, the method comprising:

-   -   providing a MOSFET device having a silicon gate electrode 5         formed on a gate dielectric 4,     -   providing a nickel layer 12 in contact with said semiconductor         gate electrode 5, and     -   performing a silicidation process to form a Ni₃Si₂ silicide 15         at least at the interface with the gate dielectric 4.

In one embodiment, a silicidation process comprises:

-   -   performing a first thermal process step to have the         semiconductor gate electrode 5 react with the nickel 12 thereby         forming a gate electrode stack comprising NiSi 13 and a         metal-rich silicide phase 14, preferably Ni₂Si,     -   selectively removing unreacted nickel (this NiSi process window         is indicated in FIG. 6 with the dotted triangle 12), and     -   performing a second thermal process step to have the gate         electrode stack comprising NiSi 13 and a metal-rich silicide         phase 14, preferably Ni₂Si, converted into a Ni₃Si₂ silicide         phase 15 at least at the interface with the gate dielectric 4.

Preferably after the first thermal step the gate electrode comprises a stack of NiSi 13 in contact with the gate dielectric 4 and Ni₂Si 14 in contact with the NiSi 13. Preferably this silicide stack 13, 14 is fully converted in Ni₃Si₂ silicide 15.

FIGS. 7 a-d illustrate by means of cross-sectional views this method of fabricating a nickel fully-silicided-gate device comprising a Ni₃Si₂ silicide phase 15 at least at the interface with the gate dielectric 4.

FIG. 7 a shows a schematic cross-sectional view of Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). The MOS device 1 comprises a gate stack formed on a semiconducting layer or substrate 2. The gate stack comprises a gate dielectric 4 in contact with this semiconducting layer 2 and a gate electrode 5 in contact with this gate dielectric 4. The gate dielectric can be a silicon-oxide, a silicon-oxy-nitride (Si_(x)O_(y)N_(1-x-y)), a high-k dielectric (k>3.9) such as Hafnium or Aluminum silicates or oxides, e.g. HfO₂, Al₂O₃. Preferably the gate dielectric 4 is a silicon-oxide or a Hafnium-based dielectric, such as HfSi_(x)O_(y). The gate electrode 5 comprises a semiconductor layer comprising Si, Ge or SiGe. This semiconductor layer is preferably silicon. This silicon can be in a monocrystalline, polycrystalline or amorphous state. The thickness of the semiconductor gate electrode 5, representing the amount of semiconductor material available, can be determined in view of the desired composition of the later-formed silicide. The MOS device can further comprise spacers 8 formed adjacent the gate stack 4, 5. Spacers can be formed from silicon-oxide, silicon-oxy-nitride (Si_(x)O_(y)N_(1-x-y)), silicon nitride, silicon carbide or from a combination of these materials. At opposite sides of this gate stack 4, 5 source/drain junction regions are formed, typically as a combination of lower doped extension regions 6, 7 underneath the spacers 8 and higher doped connection regions 9, 10 adjacent these extension regions and aligned to the spacers 8. Dielectrically coupled to the gate stack, a channel region 11 is present in the semiconductor layer 2, which in this case is the semiconductor substrate itself 2. The active area region (not indicated) in which the transistor 1 is formed, is separated from other active area regions by so-called field regions 3 formed by known techniques such as Local Oxidation of Silicon (LOCOS) or Shallow Trench Insulation (STI).

FIG. 7 b shows a schematic cross-sectional view of the semiconductor device 1 after the nickel layer 12 deposited overlying the substrate. The nickel layer 12 is in contact with the exposed semiconductor gate electrode 5 and, in this example, with the exposed regions of the semiconductor layer 2.

FIG. 7 c shows a schematic cross-sectional view of the semiconductor device 1 after the first thermal step and selective etch of the silicidation process. A first thermal process step is performed using Rapid Thermal Processing (RTP) to have the nickel 12 react with the contacted semiconductor material 5. Silicide 16 will form in the connection regions 9, 10 and a stack of NiSi 13 and Ni₂Si 14 is formed in the gate electrode 5. Different sources of thermal energy known in the art, such as spike anneal, laser anneal, and furnace anneal can be applied to provide the thermal budget of the first thermal step. The nickel 12 to silicon 5 ratio x/y is selected between 1 and 2, preferably between 1.5 and 2, while the thermal budget of the first thermal step RTP1 is selected between 360° C. 30 s and 380° C. 30 s. After the first thermal treatment step the remaining unreacted metal 12 is selectively removed, preferably by a wet etch, e.g. of about 5 minutes, e.g. using dilute HNO₃. In particular any excess metal present near the MOSFET device will also be removed during this removal step.

Thereafter, a second thermal process step is performed to convert the NiSi/Ni₂Si stack 13, 14 in a stack comprising Ni₃Si₂ 15 at least at the interface with the gate dielectric 4. The thermal budget of the second thermal step RTP1 is preferably above 500° C. 30 s. 

1. A metal-oxide-semiconductor field effect transistor (MOSFET) comprising a gate electrode and a gate dielectric, wherein the gate electrode comprises Ni₃Si₂ at least at an interface with the gate dielectric.
 2. The MOSFET of claim 18, wherein the Ni₃Si₂ further comprises a dopant for modulating the work function of the Ni₃Si₂.
 3. The MOSFET of claim 19, wherein the dopant is selected from the group consisting of phosphorous and boron. 